Method of producing a composite carbide and nitride case on steel articles



March 2l, 1939.

METHOD oF PRoDucING A coMPo Patented Mar. 21, 1939 PATENT OFFICE 'METHOD OF PRODUCINC A COMPOSITE CARBIDE AND NITRIDE CASE ARTICLES oN STEEL RoyseiJ. Cowan, Toledo, Ohio, assignor to Snrface Combustion Corporation, Toledo, Ohio, a corporation oi' New York Application December 31, 1938, Serial No. 248,753

Claims- It has heretofore been proposed to use a carburizing gas as a diluent for a nitriding gas for the purposev of rendering' an ordinary nitride case less brittle but insofar as I am'aware there has hitherto not been any way'of utilizing a nitriding gas like ammonia for imparting to anordinary carbide case the desirable characteristics of a nitride case. It is, therefore, the lobject of the present invention to provide a commercially practical Way of utilizing a nitriding gas for imparting to a carbide case the desirable characteristics of a' nitride case and withal to provide a method which shall permit a variety of composite nitride and carbide cases to be obtained lby simply varying, within certain limits, the rel.

ative proportions of the nitriding and carburizing gases utilized and/ or by varying the furnace temperature.

The present invention may be said to be based on the discovery that by introducing the nitrid' ing gas into the chamber of a heat-treating furnace in a certain way it'becomes possible to produce a carbide case with certain of the desirable l characteristics of a nitrided case. The case produced by the present invention is, therefore, a modied carburized case.

In accordance with the present invention the nitriding' gas is introduced into the soaking zone `of a continuous heat-treating furnace through a series of inlets the first of which is located adjacent the point where the heating-up zone may b e said to end and the soaking zone to begin.

. Moreover, the best results are obtained when the nitriding gas is introduced below its break-down temperature. It will, of course, be understood vthat by the term heating-up zone is meant that allowed'to remain in the furnace whereas the chemical composition of the case depends on the furnace temperature and on the proportions of carburizing and nitriding gases used.

In general it may be'said that the operating range as regards temperature is from about 1200 to 1600 deg.. F. and as regards the proportions l, of c'arburizing andnitriding gases the range is from about 10% to about 50% .of nitriding gas. However, the preferred temperature is about 1450 deg. F. and the preferred proportions of said gases are about three volumes of nitriding gas to seven volumes of carburizing gas. The manner in which the carburizing gas is introduced into the furnace is of more or less secondary im- .portance as 4will more fully appear hereinafter.

drawing for a graphic illustration of certain feaproduce a case which is somewhat similar to that produced when using the well known cyanide case-hardening bath.

The method shown in Fig. 3 will produce a case having a somewhat higher carbide concentration than is produced by the method of Fig. 1, the case in this respect being similar to that produced when using a so-called activated cyanide bath. v

The method shown in Fig. 2 will produce a somewhat higher nitride concentration than the method shown in Fig. 1.

The nitriding gas will ordinarily be introduced into the chamber through relatively small openings and under a pressure somewhat above atmospheric pressure. Although it appears to be more or less immaterial whether the nitriding gas is introduced through the top or side walls or bottom of the chamber, nevertheless the preferred place of introduction is through the top -in a direction crosswise of the path of travel of the articles through the chamber. `As previously vmentioned the nitriding gas is preferably introduced below its break-down temperature and this is conveniently accomplished by the use of air cooled inlet pipes.

in the spirit of the invention to use other nitrogen-containing'compounds which like ammonia have the property of releasing nitrogen when heated,v such as formamide, hydrocyanic acid, nitrobenzene, urea and pyridine. It is also Within the spirit of the invention to use other well known carburizing gases such as coke oven gas, charcoal gas, and the common hydrocarbon gases such as natural gas, propane and butane either alone or admixed with air or combustion products.

This application is a continuation in part of my application Serial No. 169,004 filed October 14, 1937.

What I claim is: v

1. The method of simultaneously utilizing a nitriding gas and a carburizing gas for producing a composite carbide and nitride case on steel articles which are being progressed through a furnace chamber maintained at a selected temperature between 1200-1600 deg. Fahr. which comprises initially introducing the nitriding gas into said chamber at a zone intermediate the ends of said chamber, and introducing the carburizing gas into said chamber independently of the nitriding gas.

2. The method of simultaneously utilizing a nitriding gas and a carburizing gas for producing a composite carbide vand nitride case on steel articles which are being progressed through a furnace chamber maintained at a selected temperature between 1200-1600 deg. Fahr. which comprises initially introducing the nitriding gas into said chamber at a zone beginning at about that point where the incoming articles first attain the maximum temperature prevailing in said chamber, and introducing the carburizing gas into said chamber independently of the nitriding gas.

3. The method of simultaneously utilizing a nitriding gas and a carburizing gas for producing a composite carbide and nitride case on steel articles which are being progressed through a furnace chamber maintained at a selected temperature between 1200-1600 deg. Fahr. which comprises initially introducing the nitriding gas into said chamber at a zone beginning at about that point where the incomingarticles first attain the maximum temperature prevailing in said chamber, and introducing the carburizing gas` into said chamber independently of the nitriding gas, the amount of nitriding gas thus introduced into said chamber comprising from about 10% to 50% of the total volume of the gases introduced into said chamber.

4. The method of simultaneously utilizing a nitriding gas and a carburizing gas for producing -a composite carbide and nitride case on steel articles which are being progressed through a furnace chamber maintained at a selected temperature between 1200-1600 deg. Fahr. which when the nitriding gas is ammonia and the carburizing gas is manufactured city gas.

5. In the method speciiied in claim 2, said nitriding gas being introduced below its breakfdown temperature.

6. That method `of producing av case on steel articles while they are being progressed through a chamber maintained at a selected temperature Abetween 1200-1600 deg. F. and having a heatinging gas, and introducing and maintaining va carburizing gas in said chamber.

7. Inthe method specified in claim 6, said nitriding gas being introduced into said soaking zone at a controlled temperature which is below its break-down temperature.

8. In the method specified in claim 6, said nitriding gas being introduced into said soaking zone at a controlled temperature which is below its .break-down temperature and being initially discharged in a direction across thepath of travel of the articles through the soaking zone whereby the nitriding gas initially ilowstoward the articles in that zone.

9. 'I'hat method of producing a case von steel articles while they are being progressed through a chamber maintained at a selected temperature between 1200-1600 deg. F. and having a heating- .up zone wherein the articles are first heated up to temperature and having a soaking zone where-4 in the articles after leaving the heating-up zone y are maintained at temperature, which comprises introducing a multiplicity of streams of nitriding gas into said soaking zone at points along the length of the latter, said streams being initially discharged in a direction across .the path of travel of the articles through the soaking zone and being introduced into said soaking zone at a controlled temperature, whereby the articles in passing through said soaking zone are repeatedly subjected to the action of fresh nitriding gas, and introducing and maintaining a carburizing gas in said chamber.

10. In the method specified in claim 6, the carburizing gas being introduced into said chamber at the heating-up zone thereof.

1l. In the method specified in claim 6, the mixture of gases in said chamber being bled therefrom adjacent the article-leaving end of the soaking zone. f

l2. In the method specied in claim 6, the carburizing gas being introduced into the chamber at a plurality of points along the length of the soaking zone.

13. In' the method specified in claim 6, the carburizing gas being introduced into the soaking zone of said chamber and the mixture of gases in the chamber being bled therefrom at a point within the heating-up zone thereof.

14. In the art of case hardening steel articles in a heat treating chamber, the steps of passing said articles iirst vthrough a heating-up zone and then through a soaking zone in said chamber, maintaining said soaking zone at a temperature between 1200 deg. F. and 1600 deg.\F., introducing and maintaining within said soaking zone a carburizing gas and to Asaid gas adding at a plurality of points within said soaking zone a nitriding gas initially discharged in a direction across the path of travel of said articles through said soaking zone.

15. In the art of case hardening steel articles in a heat-treating chamber by means of gases, the

steps of passing said articles progressively through a heating-up zone and a.v soaking zone` of said chamber, maintaining said soaking zone at a temperature between 1200 deg. F. and 1600 deg. F. and, during the passage of said articles throughv said chamber, subjecting said articles 

